What do we think we know about Cavity Walls?

  • Building Design Expert
  • 10 years ago

What can we find exciting on the subject of external wall construction? After all we’ve been building them since we moved out of the cave. Don’t know what your last cave was like but mine was very damp. Mainly due to all the water. Well of course it was due to the water; after all the ground is full of it. Well at least most of the time in the UK. As designers of buildings we spend half the time trying keep our buildings warm, and the other three quarters trying to keep them dry.

Water has a tendency to come up from the ground, in from the outside, and even comes from the inside – trying to get out; well it did with my last leaking pipe. Our buildings are constantly attacked from all directions and spheres. So what’s to be done?

Ground Water –
It’s there. We know it’s there, and we try our damnedest to surround the bits of the building we elect to dip into the ground, with a waterproof separating layer and trust that the contractor will take as much care installing that layer as it was thought about during the design process. The DPM, or Dynamically Porous Material, as is so often installed, we would love, if say the joints were lapped, and taped, and it totally encapsulated the bit of the building we are trying to keep dry.

Taping always seems to be a problem for a large percentage of building contractors. The last job (that went stunningly wrong) that I went to, saw a polythene layer that was just about lapped (about 20mm) and was being attempted to be held together with gaffer tape, and failing miserably, as the gaffer tape was more stuck to itself rather than to the DPM. Then, as if to add insult to injury; a retaining timber was securely held in place using 100mm hammer fixings straight back through the DPM into the sub-ground that is full of the water we were trying keep out.

If I were trying to be fair here I would say that this is not the norm, but unfortunately it is still all too common and does nothing to ward off the fears that some of us professionals have about some of the building community, never mind the man on the top deck of the Clapham omnibus. When you see results like this it’s no wonder that guy keeps on travelling.

Water from the skies –
Cavity wall construction. Lets be honest the Romans probably invented it. So that’s another thing they did for us then. Another wonderful principle of invention for building construction; allowing us to insulate as well as keeping out all that water. Perfect. What more do we need. Next subject. Wouldn’t that be great, but for the 2 part input: the Design and the Build. Not all building designers are angels (only some of us); but, for example, it’s the execution in construction of a fairly standard insulated cavity wall that any competent brickie should do hands tied behind his back. Unfortunately many walls turn out as though they have been built in just this way.

On the part of the Building Design Expert we always assume the external leaf of the cavity wall will be porous – in theory and  in practice. We assume that in heavy rain there will be water positively cascading down the inside face of the external leaf (that’s, within the cavity). Obviously much depends on the material – does it?. No, brick, stone, block, whatever, it’s POROUS. If it doesn’t come through the material itself, it will come through the joints. The trick being to keep that water from bridging the cavity over to the insulation and the sacred inner leaf. Keep off!! Indeed, but does the designer invite the angst by then specifying things like the inverted cavity tray, or any cavity tray for that matter? Another inherent problem for our brickie!!

What a fantastic invention cavity trays are – standard, inverted, gilded, or with flashing LEDs. OK the inverted one was a bit of a joke. On a sectional construction drawing they look great. I love them. Wouldn’t it be wonderful if we could just leave them on a drawing – a piece of technical artistry to take to the Tate Modern. Alas, they have to be built. Now I hate them. I hate they cannot be dressed into the inner leaf evenly. I hate that it’s junction with the outer leaf fills up with dropped mortar. Surely not dropped mortar? Sadly yes. I’ve been at this job for a decade or three now, and still the penny hasn’t dropped with the brickie – only the mortar.

So I watch the next series of Dragon’s Den in eager anticipation of someone wanting a £100K for 4% of their company which has totally reinvented the cavity wall without a cavity, or DPC. – DPCs? Now don’t get me started.

Wouldn’t it be great? – floor level and external ground level are the same and there’s no inverted cavity tray. So how do we do that then? No, not because the builder hasn’t read the drawing and so left it out. Builders reading drawings!? – We go back to my last blog on the business of “Communication”. You can lead a brickie to a cavity, but you can’t necessarily make him do anything other than he normally does. Most of the time – therein lies the problem.

Here’s a typical detail that you might find on any job – large or small. It shows the external ground level to be the same as the new finished floor level. A prime candidate for an ‘inverted cavity tray’. This is a good solution on a drawing, but not necessarily on site. Cavity trays are notoriously difficult to install. Double that for an inverted one!

DETAIL ‘A’ click to enlarge

 Detail ‘A’ shows the inverted cavity tray option, whilst Detail ‘B’ might be a little more expensive, but taking care, with the right specification of materials, and to seal all the joints, and of course with a willing tradesman prepared to try something different from his last job – we may just have a viable, and less troublesome alternative.

Now I am not going to claim this as my invention. Certainly not. Certain manufacturers have been promoting this principle for a long time, and we have been ignoring it for even longer, mainly because it costs more. – COST: the bain of the designers life. How many failed construction details were driven by a need to cut costs? Maybe there’s another blog on the horizon!?

We would much rather do it cheaper; where the brickie leaves it out altogether because it’s too difficult, or he manages to put it it in one way or another, and we have water bridging to the inner leaf due to mortar droppings, or poor dressing. And we all know how poorly some builders can dress; not to mention architects – although I just did.

DETAIL ‘B’ – click to enlarge

We are looking here at a host of workmanship issues. How many brickies have you met who will willingly bed the DPC on a 5mm mortar bed, then put the next brick on a 5mm bed over it. They look at me as though I’ve just been released from the asylum when I come out with that one. In fact I have now become almost immune to the next line which is “That’s not how we normally do it”.

Of course we all know they loose lay the DPC over the top most brick course and then bed the next brick on a 10mm bed over that. OK there’s the first movement joint. Only we didn’t actually want a movement joint there did we? To be fair there must be hundreds of thousands of buildings with this detail – up and down the country. They will be decades old and they will still be being used for what they were designed for in the first place. So where’s the problem?

The problem lies in these buildings developing defects such as cracking to plaster and masonry due to differential movement. There may also be pockets of damp occur under certain conditions such as a lot of rain, with the wind in the wrong direction, snow build up, the change from BST to GMT – who knows?

These are just examples, and all by varying degrees from imperceptable to very perceptable. Surveyors note it down on building reports, and when, and if it gets really bad, the building owner will have it repaired – as best as can be done. But of course the real damage was done during construction.

Old pessimist that I might appear; what can we do? Well if the building owner won’t cover the fees for ‘site inspections’, the answer is not much on that front. The real answer lies with educating the building contractor, the tradesman, in the real life consequences of what they do.

That will be an interesting one!

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